Wide band LNA with noise canceling

ABSTRACT

Techniques to improve low noise amplifiers (LNAs) with noise canceling are described. LNA includes a first and a second amplifier which work together to noise cancel the noise generated at an input stage circuit. The input stage circuit receives an RF signal and is characterized by a first node and a second node. The first amplifier converts a noise voltage at the first node into a first noise current at an output of the first amplifier. The second amplifier is directly coupled to the output of the first amplifier and provides noise canceling by summing the first noise current with a second noise current generated by the second amplifier as a function of the noise voltage at the second node. The proposed techniques eliminate the need for large ac coupling capacitors and reduce the die size occupied by the LNA. The elimination of ac coupling capacitors between amplification stages of the LNA allows current reuse resulting in reduced current consumption.

CLAIM OF PRIORITY UNDER 35 U.S.C. §119

The present application for patent claims priority to ProvisionalApplication No. 61/303,589, entitled “Noise Cancelling Wideband LNA”filed Feb. 11, 2010, and assigned to the assignee hereof and herebyexpressly incorporated by reference herein

TECHNICAL FIELD

The present disclosure relates to electronics and more particularly tothe field of wide band low noise amplifiers for radio frequencyintegrated circuits.

BACKGROUND

Mobile, wireless, and/or handheld portable devices increasingly becomemultifunctional communication devices. These handheld portable devicesintegrate an increasingly wide range of functions for handling aplurality of wireless communication services. For example, a singlehandheld portable device may include an FM receiver along with GPS,CDMA, Wi-Fi, WiMAX, CDMA2000 and 3G receivers.

FM receivers require wideband low-noise amplifiers (LNAs) as theyoperate from 76 MHz to 108 MHz. Specifications requirements for FMreceiver sensitivity include a wide band LNA with sufficient gain and anoise figure (NF) that is below 3 dB. This NF might be easily achievablefor LNAs using source degenerating inductors.

Moreover, source impedance matching is usually required to limitreflections on an antenna or to avoid alterations of the characteristicsof any RF filter preceding the LNA. An LNA exploiting an inductorachieves such requirements, but only in a narrow frequency band aroundresonance.

Inductors tend to occupy significant die area. As in newer CMOStechnologies the area costs increase, area consuming inductors increasethe overall device cost. FM receivers are considered auxiliary productsin a multifunctional communication device and they must occupy verysmall die area so they do not impact the overall device cost. Thus, theuse of inductors for FM receivers is prohibitive.

Alternative solutions for impedance matching have to be considered toachieve power matching at the input of the LNA. Alternative solutionsfor impedance matching have to be considered to achieve power matchingat the input of the LNA. One such solution may include adding inputresistive termination to a common source gain stage of the LNA. Analternative solution is to use a common gate as input stage of the LNA.Yet another solution is to use a shunt feedback common source stage asinput stage of the LNA. These methods would provide a good powermatching but may greatly degrade the NF. Most LNAs based on resistivetermination provide good power matching but greatly degrade the NF.

To decouple impedance matching from noise figure various circuittechniques that include noise canceling have been proposed. Most ofthese circuit techniques are based on the following principle ofoperation: An RF signal appears at a first node while a replica of theRF signal, proportional to the RF signal, appears at a second node.However, the thermal noise contribution due to the input stage of theLNA appears with opposite polarity at each node. Thus, noisecancellation may be achieved by summing the RF signal and its replica.

Various such topologies have been shown in the literature. Yet, there isalways a need to improve them and achieve even smaller die size and evenlower power consumption, particularly for auxiliary components such asFM receivers.

FIG. 1 shows an implementation of an LNA with noise canceling for an FMreceiver.

The LNA includes input stage circuit 12, first amplifier 14 and secondamplifier 16. Input stage circuit 12, includes MOS transistor M₁ in acommon gate configuration coupled to load resistor R_(L). Firstamplifier 14 comprises a pair of complimentary MOS transistors, M2 p andM2 n, in a common source configuration and is ac-coupled to the secondamplifier through capacitors C_(3ac) and C_(4ac). Each of thesecapacitors along with each of resistors R_(1DG) and R_(2DG),respectively, forms a high-pass filter for the signal appearing at theoutput of first amplifier 14. Typically, R_(1DG) is equal to R_(2DG).The impedance at the input of the second amplifier, and morespecifically at the source node of transistors M3 n and M3 p, is 1/gm inparallel with R_(1DG) or R_(2DG). The transconductance of transistors M3n and M3 p is defined as gm. Given reasonable transistor size andbiasing current, the impedance looking into the input of the secondstage may be well below 1K ohm. Within the FM band, (76 MHz˜108 MHz),the signal at the output of the first amplifier has to pass through thehigh-pass filter with minimum loss, which dictates that the high passcorner of the high-pass filter has to be much lower than the loweroperating frequency of the FM band (76 MHz). Thus, ac couplingcapacitors C_(3ac) and C_(4ac) have to be quite large to meet thisrequirement.

The use of large capacitors is undesirable not only because they occupysignificant die area but also because they do not allow sharing orreusing of current between the different amplification stages of theLNA; thus, resulting in noise canceling solutions with a high currentconsumption LNA.

It is desirable to design a noise canceling LNA that does not requireeither a large inductor or large capacitors and that is suitable for lowcurrent consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an implementation of an LNA with noise canceling for an FMreceiver.

FIG. 2 is a schematic diagram of an exemplary embodiment of an LNA withnoise canceling.

FIG. 3 is a simplified schematic diagram illustrating the noisecanceling principle for the LNA exemplary embodiment of FIG. 2.

FIG. 4 is a schematic diagram of another embodiment of an LNA with noisecanceling.

DETAILED DESCRIPTION

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments.

The detailed description set forth below in connection with the appendeddrawings is intended as a description of exemplary embodiments of thepresent invention and is not intended to represent the only embodimentsin which the present invention can be practiced. The detaileddescription includes specific details for the purpose of providing athorough understanding of the exemplary embodiments of the invention. Itwill be apparent to those skilled in the art that the exemplaryembodiments of the invention may be practiced without these specificdetails. In some instances, well known structures and devices are shownin block diagram form in order to avoid obscuring the novelty of theexemplary embodiments presented herein.

Those of skill in the art would understand that information and signalsmay be represented using any of a variety of different technologies andtechniques. For example, data, instructions, commands, information,signals, bits, symbols, and chips that may be referenced throughout theabove description may be represented by voltages, currents,electromagnetic waves, magnetic fields or particles, optical fields orparticles, or any combination thereof.

The present disclosure is directed to various embodiments of an improvedLNA with noise canceling. The disclosed solutions eliminate the need forlarge ac coupling capacitors thus reducing the die size occupied by theLNA. Furthermore, the elimination of ac coupling capacitors betweenamplification stages of the LNA allows current reuse thus resulting inlow current consumption.

According to the present disclosure, the LNA incorporates an input stagecircuit, a first amplifier and a second amplifier. The input stagecircuit receives an RF signal and it is biased to provide matching to asource impedance. The first amplifier amplifies a first voltageappearing at the input of the LNA. The first voltage has a first noisevoltage component due to the noise of the input stage devices. The firstamplifier converts the first noise voltage component into a first noisecurrent and feeds the first noise current to the second amplifier.

A second amplifier amplifies a second voltage. The second voltage isproportional to the first voltage and has a second noise voltagecomponent. The second noise voltage component is proportional and ofopposite phase to the first noise voltage. The second amplifier convertsthe second noise voltage component into a second noise current and addsthe second noise current to the first noise current so that the firstnoise current and the second noise current cancel each other at theoutput of the LNA.

The first amplifier is directly coupled, dc coupled, to the secondamplifier. As a result, the disclosed solution uses smaller accapacitors in the LNA when compared with previous solutions such as thesolution shown in FIG. 1.

FIG. 2 is a schematic diagram of an exemplary embodiment of an LNA withnoise canceling. LNA 20 includes input stage circuit 22, first amplifier24 and second amplifier 26. Input stage circuit 22, includes MOStransistor 202 (M₁) in a common gate configuration coupled to loadresistor 222 (R_(L)). First amplifier 24 includes a pair ofcomplementary MOS transistors 250, 252 (M_(PCS), M_(NCS)) and capacitors242 and 244. Second amplifier 26 comprises MOS transistor 260 (M_(NSF))and capacitor 262. MOS transistor 260 is in a source followerconfiguration.

The gates of MOS transistors 250 and 252 are coupled to the input of theLNA, node 200, through capacitors 242 and 244, respectively. The sourceof first MOS transistor 250 of the pair of complementary MOS transistorsis coupled to the positive supply. The drain of first MOS transistor 250of the pair is coupled to the drain of second MOS transistor 252 of thepair and to the source of MOS transistor 260. The source of second MOStransistor 252 of the pair is coupled to ground.

LNA 20 further comprises MOS transistor 266 (M_(PSF)) and resistor 212(R_(DG)). The gate of MOS transistor 266 is coupled to the inputterminal of the LNA through capacitor 264, the source of MOS transistor266 is coupled to the positive power supply and the drain of MOStransistor 266 is coupled to the output of the LNA. MOS transistor 266is used as a third amplifier to improve the overall gain of the LNA.Resistor 212 has one terminal coupled to the source of MOS transistor260 and another terminal coupled to ground. Resistor 212 is used as adegeneration resistor to MOS transistor 260.

First amplifier 24 senses a first noise voltage appearing at node 200.Node 200 is the input of the LNA. The first amplifier converts the firstnoise voltage into a first noise current at node 220. Node 220 is theoutput of the first amplifier.

Second amplifier 26 senses a second noise voltage appearing at node 210.Node 210 is the output of the input stage circuit. The second noisevoltage is proportional to the first noise voltage. Second amplifier 26converts the second noise voltage into a second noise current, whichappears at node 240. Node 240 is the output of the second amplifier andis also the output of the LNA.

Second amplifier 26 sums the first and the second noise currents at theoutput of the LNA in such a way that the first and the second noisecurrents substantially cancel each other. As a result, the noise due tothe input stage circuit is substantially canceled at the output of theLNA.

The transconductance of the first amplifier is designed as a function ofthe transconductance of the second amplifier in order to set the firstnoise current equal and of opposite sign to the second noise current.

The first amplifier is directly coupled, dc coupled, to the secondamplifier. As a result, the LNA uses smaller ac capacitors when comparedwith previous solutions such as that shown in FIG. 1.

A simplified schematic view of the LNA is shown in FIG. 3 to furtherillustrate the noise canceling mechanism. FIG. 3 is a simplifiedschematic diagram illustrating the noise canceling principle for the LNAexemplary embodiment of FIG. 2.

NMOS transistor M_(2n) represents the transconductance gm2 of the pairof complementary MOS transistors 250 and 252 used as the first amplifierin FIG. 2. NMOS transistor M_(3n) represents the transconductance gm3 oftransistor 260 used as the second amplifier in FIG. 2.

Noise current source 270 represents the noise current In due to MOStransistor 220 included in the input stage circuit of FIG. 2. Resistor280 has a value equal to r_(ds) and represents the total outputimpedance of the pair of complementary MOS transistors 250 and 252.

The noise current of MOS transistor 220 which is modeled by the currentsource 270, flows into node 200 but out of node 210. This creates twofully correlated noise voltages with opposite phases. The first noisevoltage_(V1), at node 200, equals to alpha*In*Rs and the second noisevoltage_(V2), at node 210, equals to -alpha*In*R_(L). These two voltagesare converted to currents by the first and the second amplifier,respectively. By properly selecting transconductances g_(m2) and g_(m3),the noise contributed by MOS transistor 220 can be cancelled at theoutput of the LNA. On the other hand, the signal voltages at nodes 200and 210 are in phase, resulting in constructive addition at the output.The condition for complete noise cancellation is derived by thefollowing equation as:

${\alpha\;{I_{n} \cdot R_{S} \cdot g_{m\; 2n} \cdot \frac{r_{ds}}{r_{ds} + \frac{1}{g_{m\; 3}}}}} = {\left. {\alpha\;{I_{n} \cdot R_{L} \cdot \frac{1}{r_{ds} + \frac{1}{g_{m\; 3}}}}}\rightarrow{{R_{s} \cdot g_{m\; 2n}}r_{ds}} \right. = R_{L}}$

Assuming a degeneration factor β:

$\beta \equiv \frac{g_{m\; 3}r_{ds}}{1 + {g_{m\; 3}r_{ds}}}$The gain of the LNA is

$A_{v} = \left. {{g_{m\; 1} \cdot R_{L} \cdot \frac{g_{m\; 3}}{1 + {g_{m\; 3}r_{ds}}}} + {g_{m\; 2n} \cdot \frac{g_{m\; 3}r_{ds}}{1 + {g_{m\; 3}r_{ds}}}}}\Rightarrow{2\; g_{m\; 2n}\beta} \right.$and the Noise Factor is:

${NF} = {\left. {1 + \frac{\left\lbrack {\frac{4{KTR}_{L}}{R_{DG}^{2}} + \frac{4{{KT} \cdot {NEF}}}{g_{m\; 3}R_{DG}^{2}} + \frac{4{KT}}{R_{DG}} + {4{{KT} \cdot {NEF} \cdot g_{m\; 2n}}}} \right\rbrack \cdot \beta^{2}}{4{{KTR}_{S} \cdot \frac{1}{4} \cdot 4}{\beta^{2} \cdot g_{m\; 2n}^{2}}}}\Rightarrow{1 + \frac{1}{\left( {g_{m\; 2}R_{DG}} \right)} + \frac{NEF}{\left( {g_{m\; 2}R_{S}} \right)\left( {g_{m\; 2}R_{DG}} \right)\left( {g_{m\; 3}R_{DG}} \right)} + \frac{1}{\left( {g_{m\; 2}R_{S}} \right)\left( {g_{m\; 2}R_{DG}} \right)} + \frac{NEF}{\left( {g_{m\; 2}R_{S}} \right)}}\Rightarrow{1 + \frac{NEF}{\left( {g_{m\; 2}R_{S}} \right)}} \right. = {1 + \frac{2{\beta \cdot {NEF}}}{A_{V}R_{S}}}}$

Degeneration factor β is typically lower than one and helps to provide adesign tradeoff between power consumption and noise figure.

FIG. 4 is a schematic diagram of another embodiment of an LNA with noisecanceling.

LNA 40 includes input stage circuit 42, first amplifier 44 and secondamplifier 46. Input stage circuit 40, includes MOS transistor 402 (M₁)in a common gate configuration coupled to load resistor 422 (R_(L)).First amplifier 44 includes a first pair of complementary MOStransistors 450, 452 (M_(PCS), M_(NCS)) and capacitors 442 and 444.Second amplifier 46 comprises a second pair of MOS transistors 466, 460(M_(PSF), M_(NSF)) and capacitors 462 and 464.

The gates of the first pair of complementary MOS transistors 450, 452(M_(PCS), M_(NCS)) are coupled to input terminal 400 of the LNA throughcapacitors 442 and 444, respectively. The source of MOS transistor 450is coupled to the positive supply and the drain of MOS transistor 450 iscoupled to the source of MOS transistor 466. The drain of MOS transistor466 is coupled to the drain of MOS transistor 460 and to the output ofthe LNA. The source of MOS transistor 460 is coupled to the drain of MOStransistor 452. The source of MOS transistor 452 is coupled to ground.

The gates of the second pair of complementary MOS transistors 466, 460(M_(PSF), M_(NSF)) are coupled to an output of input stage circuit 42through capacitors 462 and 464 respectively.

First amplifier 44 senses a first noise voltage appearing at node 400.Node 400 is the input of the LNA. The first amplifier converts the firstnoise voltage into a first noise current. The first noise current is adifferential signal appearing at nodes 420 and 424. Nodes 420 and 424are the outputs of the first amplifier.

Second amplifier 46 senses a second noise voltage appearing at node 410.Node 410 is the output of the input stage circuit. The second noisevoltage is proportional to the first noise voltage. Second amplifier 46converts the second noise voltage into a second noise current, whichappears at node 440. Node 440 is the output of the second amplifier andis also the output of the LNA.

Second amplifier 46 sums the first and the second noise currents at theoutput of the LNA in such a way that the first and the second noisecurrents substantially cancel each other. As a result, the noise due tothe input stage circuit is substantially canceled at the output of theLNA.

The transconductance of the first amplifier is a function of thetransconductance of the second amplifier in order to set the first noisecurrent equal and of opposite sign to the second noise current.

The first amplifier is directly coupled, dc coupled, to the secondamplifier. As a result, the LNA uses smaller ac capacitors when comparedwith previous solutions such as that shown in FIG. 1.

The first and the second amplifier are combined in a way that they sharecurrent as the current at the differential output of the first amplifieris fed directly to the sources of the second pair of MOS transistors.Current sharing results in a low current implementation when compared totraditional implementations that use ac coupling between the LNAamplification stages.

Exemplary embodiments of the disclosed LNA such as those shown in FIG. 2and FIG. 4 when used for an FM receiver demonstrate at least 30% diearea reduction over previous solutions.

The LNA provides for dc coupling between amplification stages of the LNAthus allows current reuse between a first and a second amplifier makingthe proposed solutions low power.

Those of skill would further appreciate that the various illustrativelogical blocks, modules, circuits, and algorithm steps described inconnection with the embodiments disclosed herein may be implemented aselectronic hardware, computer software, or combinations of both. Toclearly illustrate this interchangeability of hardware and software,various illustrative components, blocks, modules, circuits, and stepshave been described above generally in terms of their functionality.Whether such functionality is implemented as hardware or softwaredepends upon the particular application and design constraints imposedon the overall system. Skilled artisans may implement the describedfunctionality in varying ways for each particular application, but suchimplementation decisions should not be interpreted as causing adeparture from the scope of the exemplary embodiments of the invention.

The various illustrative logical blocks, modules, and circuits describedin connection with the embodiments disclosed herein may be implementedor performed with a general purpose processor, a Digital SignalProcessor (DSP), an Application Specific Integrated Circuit (ASIC), aField Programmable Gate Array (FPGA) or other programmable logic device,discrete gate or transistor logic, discrete hardware components, or anycombination thereof designed to perform the functions described herein.A general purpose processor may be a microprocessor, but in thealternative, the processor may be any conventional processor,controller, microcontroller, or state machine. A processor may also beimplemented as a combination of computing devices, e.g., a combinationof a DSP and a microprocessor, a plurality of microprocessors, one ormore microprocessors in conjunction with a DSP core, or any other suchconfiguration.

The steps of a method or algorithm described in connection with theembodiments disclosed herein may be embodied directly in hardware, in asoftware module executed by a processor, or in a combination of the two.A software module may reside in Random Access Memory (RAM), flashmemory, Read Only Memory (ROM), Electrically Programmable ROM (EPROM),Electrically Erasable Programmable ROM (EEPROM), registers, hard disk, aremovable disk, a CD-ROM, or any other form of storage medium known inthe art. An exemplary storage medium is coupled to the processor suchthat the processor can read information from, and write information to,the storage medium. In the alternative, the storage medium may beintegral to the processor. The processor and the storage medium mayreside in an ASIC. The ASIC may reside in a user terminal. In thealternative, the processor and the storage medium may reside as discretecomponents in a user terminal.

In one or more exemplary embodiments, the functions described may beimplemented in hardware, software, firmware, or any combination thereof.If implemented in software, the functions may be stored on ortransmitted over as one or more instructions or code on acomputer-readable medium. Computer-readable media includes both computerstorage media and communication media including any medium thatfacilitates transfer of a computer program from one place to another. Astorage media may be any available media that can be accessed by acomputer. By way of example, and not limitation, such computer-readablemedia can comprise RAM, ROM, EEPROM, CD-ROM or other optical diskstorage, magnetic disk storage or other magnetic storage devices, or anyother medium that can be used to carry or store desired program code inthe form of instructions or data structures and that can be accessed bya computer. Also, any connection is properly termed a computer-readablemedium. For example, if the software is transmitted from a website,server, or other remote source using a coaxial cable, fiber optic cable,twisted pair, digital subscriber line (DSL), or wireless technologiessuch as infrared, radio, and microwave, then the coaxial cable, fiberoptic cable, twisted pair, DSL, or wireless technologies such asinfrared, radio, and microwave are included in the definition of medium.Disk and disc, as used herein, includes compact disc (CD), laser disc,optical disc, digital versatile disc (DVD), floppy disk and blu-ray discwhere disks usually reproduce data magnetically, while discs reproducedata optically with lasers. Combinations of the above should also beincluded within the scope of computer-readable media.

The previous description of the disclosed exemplary embodiments isprovided to enable any person skilled in the art to make or use thepresent invention. Various modifications to these exemplary embodimentswill be readily apparent to those skilled in the art, and the genericprinciples defined herein may be applied to other embodiments withoutdeparting from the spirit or scope of the invention. Thus, the presentinvention is not intended to be limited to the embodiments shown hereinbut is to be accorded the widest scope consistent with the principlesand novel features disclosed herein.

What is claimed is:
 1. A low noise amplifier (LNA) comprising: an inputstage circuit to receive an RF signal and having a first node and asecond node; a first amplifier including a pair of complementary MOStransistors each coupled to the first node through a pair of capacitors,respectively, to convert a first noise voltage at the first node into afirst noise current at an output of the first amplifier; and a secondamplifier including a MOS transistor having its gate coupled to thesecond node through a capacitor, its drain coupled to an output terminalof the LNA and its source coupled to the output of the first amplifier,the second amplifier providing noise canceling by summing the firstnoise current with a second noise current generated by the secondamplifier as a function of the noise voltage at the second node.
 2. TheLNA of claim 1, where the input stage circuit sets the second noisevoltage proportional to the first noise voltage.
 3. The LNA of claim 1,where the transconductance of the first amplifier is a function of thetransconductance of the second amplifier in such a way that the firstnoise current is equal and of opposite sign to the second noise current.4. The LNA of claim 1, where the source of the first MOS transistor ofthe pair of complementary transistors is coupled to the positive supplyand where the drain of the first MOS transistor of the pair is coupledto the drain of the second MOS transistor of the pair.
 5. The LNA ofclaim 4, where the source of the second MOS transistor of the pair iscoupled to ground.
 6. The LNA of claim 1, further comprising a resistorhaving one terminal coupled to the source of the MOS transistor of thesecond amplifier and another terminal coupled to ground.
 7. The LNA ofclaim 4, further comprising a MOS transistor, where its gate is coupledto the input terminal of the LNA through a capacitor, its source iscoupled to the positive power supply and its drain coupled to the outputof the LNA.
 8. The LNA of claim 5, where the input stage is a MOStransistor in a common gate configuration coupled to a load resistor. 9.A low noise amplifier (LNA), comprising: means for receiving an RFsignal and having a first node and a second node; first means foramplifying a first voltage signal as a function of an RF signal, andincluding a pair of complementary MOS transistors each coupled to thefirst node through a pair of capacitors, respectively, to convert afirst noise voltage at the first node into a first noise current at anoutput of the first means; and second means, dc coupled to the firstmeans, for amplifying a second voltage signal as a function of the RFsignal, where the second means includes a MOS transistor having its gatecoupled to the second node through a capacitor, its drain coupled to anoutput terminal of the LNA and its source coupled to the output of thefirst means, the second means providing noise canceling by summing thefirst noise current with a second noise current generated by the secondmeans as a function of the noise voltage at the second node.
 10. The LNAof claim 9, where the means for receiving an RF signal sets the secondnoise voltage proportional to the first noise voltage.
 11. The LNA ofclaim 9, where the transconductance of the first means is a function ofthe transconductance of the second means in such a way that the firstnoise current is equal and of opposite sign to the second noise current.